Typical solar cell elements each include a light receiving surface that receives solar light and a back surface that does not receive solar light and are such that collecting electrodes (bus electrodes) for bonding to wiring members (tabs) and back surface electrodes are formed on the front and back sides. In a solar cell module, the wiring members alternatively connect the collecting electrodes formed on the light receiving surface of one solar cell element and the back surface electrodes formed on the back surface of another solar cell element adjacent to the solar cell element. For example, a good conductor, such as copper, is used for the wiring members.
A solar cell element includes a photoelectric conversion unit that performs photoelectric conversion on a semiconductor substrate, such as silicon, thin wire electrodes (grid electrodes) that collect photogenerated carriers generated in the photoelectric conversion unit, and collecting electrodes to be bonded to the wiring members to transfer the collected photogenerated carriers from the thin wire electrodes to the wiring members.
The collecting electrodes are electrodes for bonding the wiring members. A plurality of collecting electrodes are formed on the solar cell element such that they intersect with the thin electrodes. Both the collecting electrodes and the thin electrodes are formed by firing a conductive paste. The conductive paste, for example, contains glass or resin as a binder and particles of a highly conductive material, such as silver (Ag), as a filler.
For example, Patent Literature 1 discloses a solar cell module that is configured by bonding the wiring members made of, for example, copper, and the collecting electrodes formed of a conductive paste containing silver particles as described above together with solder.
An objective of the solar cell module described in Patent Literature 1 is to improve adhesion between the solar cells, in which a resin curing type conductive paste is used for the collecting electrodes, and the wiring members. The collecting electrodes are formed by firing a conductive paste in which a thermosetting resin composition and conductive powder are mixed. The thermosetting resin contains a resin whose glass transition point of 70% or more in volume percent is equal to or higher than 80° C. and equal to or lower than 200° C. The solar cell module is configured such that the wiring members coated with lead-free solder are soldered to the collecting electrodes.
Moreover, as another example, for example, Patent Literature 2 discloses a solar cell module that has an objective of suppressing the damage done to the solar cell elements due to flux by preventing contamination of the manufacturing facility caused by flux used when soldering the collecting electrodes of the solar cell elements and the wiring members together.
In the solar cell module described in Patent Literature 2, the collecting electrodes are configured to include projecting portions, which extend in a direction substantially parallel to the longitudinal direction of the electrodes, on the surface to which the connection tabs are connected, thereby suppressing the damage due to flux while using flux.
For typical solar cell modules, for example, tin (Sn)-based solder, such as Sn-3Ag-0.5Cu and Sn—Cu, is used. With the method of bonding with solder, in order to remove oxide and the like formed on each surface of the collecting electrodes and the wiring members, it is necessary to apply flux on the solder bonding portion of at least one of the surfaces of the collecting electrodes and the surfaces of the wiring members. When the wiring members and the collecting electrodes being pressed against each other are heated, oxide films on the surfaces of the collecting electrodes and the surfaces of the wiring members are removed by the reduction action of flux, thereby realizing bonding with solder.
Moreover, in contrast to the patent literatures described above, for example, Patent Literature 3 discloses a manufacturing method of a solar cell module in which the wiring members and the collecting electrodes are bonded together with resin adhesive.
In the manufacturing method of a solar cell module described in Patent Literature 3, in order to provide a manufacturing method of a solar cell module that can reduce the manufacturing time, after temporarily adhering the wiring members to the light receiving surface and the back surface of each of a plurality of solar cell elements at a temperature lower than the curing temperature of the resin adhesive, the solar cell elements are collectively subjected to thermocompression bonding at a temperature equal to or higher than the curing temperature of the resin adhesive.
In Patent Literature 3, thermosetting resin adhesive containing Ni balls coated with nickel (Ni), gold (Au), or the like or conductive particles obtained by coating plastic balls with Au or the like is arranged on the collecting electrodes and the arranged thermosetting resin adhesive is cured by heating the thermosetting resin adhesive in a state where the wiring members are pressed against the collecting electrodes, thereby realizing connection of the wiring members and the collecting electrodes. In this case, the physical connection between the wiring members and the collecting electrodes is realized by using the thermosetting resin adhesive and the electrical connection between the wiring members and the collecting electrodes is performed by coming into contact with the contained conductive particles. A strip-like film mainly made of an epoxy resin is used for the thermosetting resin adhesive.